Couplings with complementarily threaded male and female fittings are commonly used to connect brake tubes to components of a vehicle brake system. The threaded male fitting has a through passage receiving a brake tube which is flared at its end and traps the male fitting on the brake tube. The threaded female fitting has a through passage extending to a raised conic end section. As the male and female fittings are threaded together, the flared end of the brake tube is forced into sealing engagement with the conic section.
To obtain the high pressure seal required for a hydraulic brake system, both connectors must be constructed of metal, and 20-25 ft./lbs. of torque are required to threadingly engage the fittings to form an adequate face seal between the conic section of the female and the flared end of the brake tube. To achieve an adequate seal a high quality surface finish is also required on both the conic section of the female fitting and the corresponding complementary conical portion of the flared end of the brake tube.
The seal can be unreliable because of an inability to maintain close tolerances and a high quality surface finish on mass produced parts. In use, corrosion between the two metal fitting bodies and the metal brake tube complicates servicing and replacement of brake component parts and may cause seal failures.